For all practical purposes, a revolver is always going to be more reliable than a semi-auto simply because there's fewer moving parts and the ammo isn't making a transition from a magazine into the chamber. That still doesn't mean revolvers are 100% reliable, but they are more reliable than a semi-auto.
However, as has been said, with enough dirt (especially sand or dust) any gun can refuse to work. The one good thing about a revolver though, if you know you're going to have it in dust or sand, you can simply not oil the moving parts (so they won't hold the grit) and the gun will still function fine. Of course I don't recommend running them dry all the time, but you can do it if it's called for.
FWIW, IMO most qualitysemi-autos using quality magazines and good ammo are reliable enough to trust your life to, as long as the gun is properly maintained. With good equipment and proper maintenance, you should have at most only one or two malfunctions every 1,000rds, which of course is a failure rate of only one to two tenths of 1%.